The electrically writable and erasable nonvolatile memory such as an EEPROM or a flash memory can rewrite a program on board, reduce its development term, and improve its development efficiency. Therefore, the nonvolatile memory can cope with various requirements, such as small-lot multi-product production, tuning for each customer, updating of a program after shipment.
In recent years, especially, needs for a microcomputer, which consist of a nonvolatile memory and a logic circuit such as a MPU (micro processing unit), has been increased. This is because a highly-functional semiconductor device can be realized by embedding a nonvolatile memory and a logic circuit on the same semiconductor substrate. Such an embedded-type semiconductor device is widely used as a built-in microcomputer in an industrial machine, a home appliance, an automobile-mounted device or the like. Generally, a program required by a microcomputer is stored in an embedded nonvolatile memory and it is read out at any time.
Since a nonvolatile memory is excellent in portability, impact-resistance, or the like, and electrically erasable in units of block, a demand for the nonvolatile memory has been increased in recent years as a storage device for a small-sized portable information device such as a portable type personal computer and a digital still camera. In order to satisfy such a demand, it is important to reduce bit cost by reduction of a memory cell area, and various memory cell structures have been proposed in order to realize the reduction.
Currently, a nonvolatile memory cell in a practical use uses a MOS (metal oxide semiconductor) type transistor for storage where a threshold voltage varies according to trap of charges. As a charge retention mechanism of the MOS type transistor for storage, there are a floating gate type where charges are trapped in electrically isolated conductive polysilicon and a MONOS type where charges are trapped in a dielectric film having charge-trapping property such as silicon nitride.
The floating gate type is widely used for a flash memory as a program storage in a mobile phone, or a large-capacity flash memory as a data storage and it has high retention characteristic. However, it becomes difficult to secure a capacitive coupling ratio required for potential control of a floating gate according to scaling, so that a structure is complicated. It is necessary to keep a thickness of oxide film surrounding the floating gate to approximately 8 nm or more in order to suppress leakage of retention charges, and scaling for a purpose of speed-up or high integration has been approaching limitation. Since charges are trapped in a conductor, even when only a single defect as a leakage path is present in an oxide film around the floating gate, retention life extremely lowers.
On the other hand, the MONOS type generally has a tendency that it is inferior to the floating gate in retention characteristic and the threshold voltage lowers with logarithm of time. Therefore, the MONOS type has been known from long ago, but practical use thereof is confined to only some products. However, since the MONOS type is a discrete trap memory, even if there are some leakage paths, almost all retained charges are prevented from being lost, so that the system is resistant to oxide film defect. Accordingly, the MONOS type has attracted attention according to advance of scaling in recent years again, because a thin oxide film with a thickness of 8 nm or less can be applied and it is suitable to scaling; extreme lowering of retention life does not occur due to defect generated at a low probability, so that reliability prediction is easy; a memory cell structure is simple and is easily embedded with a logic circuit portion; and the like.